The objective of the proposed research is to delineate the role of humoral factors in the development and differentiation of the lung, in vivo. The approach selected was to develop a fetal rat lung organ culture system. This system allows precise manipulation of known hormones while maintaining near physiological conditions in vitro. Data collected in this manner should clearly distinguish between other tissues in the intact fetus. The proposed experiments relate primarily to the biosynthetic pathway for surfactant synthesis but can be easily adapted for the study of other biochemical or transport processes which would appear to be important to explain hormone action in lung at the molecular level. The research plan includes identifying structure-function correlations by monitoring morphological and ultrastructural changes in the organ explants following exposure to hormones.